1) Field of the Invention
The present invention relates to an improvement of a lead frame used in the production of electronic components, and to a method of producing electronic components preferably using such an improved lead frame.
2) Description of the Related Art
As is well known in this field, a lead frame is used in the production of electronic components, and various different lead frames are configured in accordance with the type of electronic component to be produced.
For example, a lead frame used to produce a quad type electronic component comprises an outside frame portion surrounding a generally rectangular central opening, and a rectangular mount portion disposed within the central opening of the outside frame portion and on which an electronic element, such as an integrated circuit (IC) chip, can be mounted. The rectangular mount portion may be supported by the outside frame portion by a bar element extending therebetween. The lead frame also comprises four lead element arrays disposed along four sides of the rectangular mount portion, respectively, the outer ends of which are integrally connected to the outside frame portion. Namely, the lead elements included in each array extend from an inner side of the outside frame portion toward the corresponding side of the rectangular mount portion. In the quad type electronic component, especially a quad flat package, the IC chip mounted on the rectangular mount portion is sealed with a suitable synthetic resin, such as an epoxy resin, by using a split mold by which the mount portion and the IC chip mounted thereon are enclosed. Accordingly, the lead frame further comprises a rectangular dam-bar element surrounding the rectangular mount portion and integrally formed with the lead elements in such a manner that each of the four sides of the rectangular dam-bar element transversely extends through the lead elements included in the corresponding array. Namely, the lead elements included in each array are connected to each other by one of the four sides of the dam-bar element. When the mounted IC chip is sealed with the resin, the dam-bar element cooperates with the split mold to prevent the resin from leaking out through clearance between the adjacent lead elements. This lead frame arrangement may be formed from a thin Ni-Fe alloy or Cu alloy plate by a photoetching process or a mechanical punching operation.
In the production of the quad type electronic component using the lead frame as mentioned above, the IC chip is mounted on the mount portion of the lead frame by using a silver or gold paste, and then a wire bonding process is performed to establish electrical connections between the IC chip and the lead elements. After the wire bonding step, the mount portion with the IC chip is enclosed by the split mold so that the dam-bar element cooperates with the split mold, the resin is then introduced into the mold, and thus the IC chip, the mount portion thereof, and the inner end portions of the lead elements are sealed in the molded resin. Thereafter, consecutively, the dam-bar element is cut to release the lead elements from the dam-bar connections, and then the lead frame with the sealed IC chip is subjected to an electrolytic-plating process to coat the lead elements with a thin Sn-Pb alloy layer. After the plating process, the lead elements are trimmed and bent by press-cutting in such a manner that the lead elements are detached from the outside frame portion of the lead frame and are bent downwardly with respect to the mount portion thereof, and as a result, the quad type electronic component (QFP) is obtained.
The trimming/bending process is performed in a press-cutting machine comprising a die assembly and a movable assembly associated therewith. In particular, the die assembly includes a bending die having a rectangular recess formed on the top face thereof and four ridge portions formed along the four sides thereof, and a cutting die surrounding the bending die and having four cutting edges disposed in parallel with the four ridge portions Each of the cutting edges is spaced apart from the corresponding ridge portion and is flush with the top thereof. The movable assembly includes a knockout having a rectangular recess formed on the lower end face thereof and four edge portions formed along the four sides thereof, a stripper surrounding the, knockout while maintaining a space therebetween, and the punch disposed in the space between the knockout and the stripper. The knockout and the stripper cooperate with the bending die and the cutting die, respectively, and the punch cooperates with both the bending and cutting dies.
In the trimming/bending process, the lead frame with the sealed IC chip is positioned on the bending die to that the sealed IC chip, i.e., the molded portion, is received by the rectangular recess thereof, the lead elements included in each array extending to bridge the space between the corresponding ridge portion and the corresponding cutting edge. Then, the knockout and the stripper are lowered until engaged with the lead elements and cooperate with the bending die and the cutting die, respectively, so that the lead elements are clamped between the edge portions of the knockout and the ridge portions of the bending die and between the stripper and the cutting die, the molded portion of the lead frame being accommodated within a cavity formed by both the rectangular recesses of the knockout and the bending die. Consecutively, the punch is then lowered until engaged with the lead elements, and cooperates with the bending die and the cutting die, whereby the lead elements are deformed downwardly to conform with an outside profile of the ridge portions of the bending die, while being detached from the outside frame portion of the lead frame.
The prior method of producing electronic components as mentioned above possesses a defect in the trimming/bending process. In particular, during the bending of the lead elements, the plating layers thereof are rubbed against by the punch and thus may be partially scraped from the surface of the lead elements. Also, the scraped fragments of the plating layers may cause a short circuits among the leads of the finished electronic components, which are very close to each other.
To alleviate this defect, i.e., that the plating layers of the lead elements are rubbed by the punch, in another method, the pre-trimmed lead elements are bent by a press machine arranged so that the rubbing action can be eliminated during the bending process. In one type of such a press machine, a cam mechanism is used to impart a cam motion to the punch, to avoid the rubbing action of the punch against the lead elements during the bending process. In another type of press machine, the punch is provided with rollers which impart a bending action to the lead elements, whereby the rubbing action can be eliminated during the bending process. These press machines however are not practical as they are complicated and expensive.
The problem mentioned above can be solved by performing the plating process after the trimming/bending process, but this approach is unadvisable as it is cumbersome and difficult to coat the trimmed and bent leads with a uniform thin Sn-Pb alloy layer by the electrolytic-plating process. Namely, during the electrolytic-plating process, the trimmed and bent leads must be directly connected to terminals connected to an electric source through a lead wire, but the formation of the uniform plating layers on the trimmed and bent leads may be damaged by the direct connections between the trimmed and bent leads and the terminals. However, by using the lead frame according to the present invention, it is possible to perform the plating process after the trimming/bending process without the hindrance as mentioned above.